The present invention relates to an improved gate design for drop gate injection molding of rubber compounds to form rubber articles. More particularly, the present invention relates to the unexpected combination of a drop gate with a lattice gate for increased gate heating efficiency and reduced cycle time while the rubber part is being injection molded.
In a typical elastomer injection molding process, uncured viscous elastomeric compound is introduced into an elongated barrel of an injection molding machine at ambient temperatures. The compound is advanced through the barrel towards a mold connected to the downstream end of the barrel, usually by either a rotating screw conveyor or a reciprocating ram or piston disposed in the barrel. As the elastomeric compound advances, it is heated by heat conduction and mechanical shear heating in the barrel to reduce its viscosity and render the elastomer more flowable and amenable to subsequent injection into the mold. Typically, the less viscous the compound, the more easily it flows through a conventional gate system and the more easily it fills a mold cavity to produce a satisfactorily molded object.
One type of conventional gate for injection molded products is a xe2x80x9cdrop gatexe2x80x9d design. This type of gate is used when a side injection system is not feasible depending upon the mold design used for certain injection molded products. With a drop gate, the elastomer is forced through a small diameter orifice and into the molded part. The majority of the shear heating occurs at the orifice area of the gate. FIG. 10 is a schematic drawing of a drop gate design 100 used to mold an article. A sprue pad 102 feeds elastomer to drop gate runners 104. From the runners 104, the elastomer flows through the drop gate orifices 106, which have a smaller diameter than the drop gate runners 104, and fills the mold cavity 108.
WO 98/56559 discloses another gate design for injection molding rubber compounds. The gate design is a lattice gate. The lattice gate minimizes differences in temperature and pressure that result in a parabolic rubber flow through the gate. This is achieved by a series of crossed flow channels. WO 98/56559 teaches replacement of the conventional flat gate of the prior art with the inventive lattice gate.
The present invention is directed to an improved method and apparatus for a drop injection gate for molding rubber. The inventive method reduces curing time and improves the heat characteristics of the rubber as it is injected.
One aspect of the invention is an improved method of drop gate injection molding rubber. The method comprising injecting a rubber into a drop gate and through drop gate runners and into a mold cavity. The rubber flows at cross angles after the rubber is injected into the gate and before the rubber enters into the drop gate runners.
Another aspect of the invention is an improved drop injection gate for injection molding rubber into a mold cavity. The gate is comprised of drop gate runners. The gate has a region adjacent to the drop gate runners comprising a first and a second plurality of spaced flow channels disposed at intersecting angles to each other to create cross direction flow of the rubber before the rubber enters into the drop gate runners.
In another aspect of the invention, the drop injection gate has more than one separate region adjacent to drop gate runners comprising the first and second plurality of spaced flow channels disposed at intersecting angles to each other.